Immunolabeling is a method for qualitative or quantitative determination of the presence of a target in a sample, wherein antibodies are utilized for their specific binding capacity. The antibodies form a complex with the target (antigen), wherein a detectable label is present on the antibody or on a secondary antibody. The detectable label is a key feature of immunolabeling, which can be detected directly or indirectly. The label provides a measurable signal by which the binding reaction is monitored providing a qualitative and/or quantitative measure of the degree of binding. The relative quantity and location of signal generated by the labeled antibodies can serve to indicate the location and/or concentration of the target. The label can also be used to select and isolate labeled targets, such as by flow sorting or using magnetic separation media. Examples of labels include but are not limited to radioactive nucleotides (125I, 3H, 14C, 32P), chemiluminescent, fluorescent, or phosphorescent compounds (e.g., dioxetanes, xanthene, or carbocyanine dyes, lanthanide chelates), particles (e.g., gold clusters, colloidal gold, microspheres, quantum dots), and enzymes (e.g., peroxidases, glycosidases, phosphatases, kinases). Ideally, the label is attached to the antibody in a manner that does not perturb the antibody's binding characteristics but enables the label to be measured by an appropriate detection technology. The choice of labels is influenced by factors such as ease and sensitivity of detection, equipment availability, background in the sample (including other labels) and the degree to which such labels are readily attached to the particular antibody. Both direct and indirect labeling of antibodies are utilized for immunolabeling. Direct labeling utilizes only a primary antibody, i.e. the antibody specific for the target, bound to the label. In contrast, indirect labeling utilizes a secondary antibody bound to the label, which is specific for the primary antibody, e.g. a goat anti-rabbit antibody. The principal differences in immunolabeling methods and materials reside in the way that the label is attached to the antibody-antigen complex, the type of label that is used, and the means by which the antibody-antigen complex is detected.
Limitations for direct labeling primary antibodies include the need for buffers free of primary amines, or carrier proteins such as bovine serum albumin (BSA), and other compounds such as tris-(hydroxymethyl)aminomethane (TRIS), glycine, and ammonium ions. These materials are, however, common components in antibody buffers and purification methods, and it may not be possible or feasible to remove them prior to the coupling reaction. In particular, many monoclonal antibodies are available only as ascites fluid or in hybridoma culture supernatants, or diluted with carrier proteins, such as albumins. Thus, direct labeling of antibodies in ascites fluid or other medias containing interfering compounds is not attainable.
The indirect immunolabeling method typically involves a multi-step process in which an unlabeled first antibody (typically a primary antibody) is directly added to the sample to form a complex with the antigen in the sample. Subsequently, a labeled secondary antibody, specific for the primary antibody, is added to the sample, where it attaches noncovalently to the primary antibody-antigen complex. Alternatively, a detectable label is covalently attached to an immunoglobulin-binding protein such as protein A and protein G to detect the antibody-antigen complex that has previously been formed with the target in the sample. Using ligands, such as streptavidin, that are meant to amplify the detectable signal also expands this cascade binding.
Indirect immunolabeling often results in false positives and high background. This is due to the fact that secondary antibodies, even when purified by adsorption against related species, nevertheless can exhibit significant residual cross-reactivity when used in the same sample. For example, when mouse tissue is probed with a mouse monoclonal antibody, the secondary antibody must necessarily be a labeled anti-mouse antibody. This anti-mouse antibody will detect the antibody of interest but will inevitably and additionally detect irrelevant, endogenous mouse immunoglobulins inherent in mouse tissue. This causes a significant background problem, especially in diseased tissues, which reduces the usefulness and sensitivity of the assay. Thus, the simultaneous detection of more than one primary antibody in a sample without this significant background interference depends on the availability of secondary antibodies that 1) do not cross-react with proteins intrinsic to the sample being examined, 2) recognize only one of the primary antibodies, and 3) do not recognize each other (Brelje, et al., METHODS IN CELL BIOLOGY 38,97-181, especially 111-118 (1993)).
To address the background problem in indirect labeling, a number of strategies have been developed to block access of the anti-mouse secondary antibodies to the endogenous mouse immunoglobulins. One such strategy for blocking involves complexing the primary antibody with a selected biotinylated secondary antibody to produce a complex of the primary and secondary antibodies, which is then mixed with diluted normal murine serum (Trojanowski et al., U.S. Pat. No. 5,281,521 (1994)). This method is limited by the necessity to utilize an appropriate ratio of primary-secondary complex. Too low a ratio of primary-secondary complex will cause a decrease in specific staining and increased background levels due to the uncomplexed secondary anti-mouse antibody binding to endogenous mouse antibodies. However, the ability of a whole IgG antibody (as was used in the referenced method) to simultaneously bind and crosslink two antigens results in too high a ratio, causing the complex to precipitate or form complexes that are too large to penetrate into the cell or tissue.
Another strategy for blocking access to endogenous immunoglobulins in the sample involves pre-incubating the sample with a monovalent antibody, such as Fab′ fragments, from an irrelevant species that recognize endogenous immunoglobulins. This approach requires large quantities of expensive Fab′ fragments and gives mixed results and adds at least two steps (block and wash) to the overall staining procedure. The addition of a cross-linking reagent has resulted in improved reduction of background levels (Tsao, et al., U.S. Pat. No. 5,869,274 (1997)) but this is problematic when used with fluorophore-labeled antibodies. The cross-linking causes an increase in the levels of autofluorescence (J. Neurosci. Meth. 83, 97 (1998); Mosiman et al., Methods 77, 191 (1997); Commun. Clin. Cytometry 30, 151 (1997); Beisker et al., Cytometry 8, 235 (1987)) and thus the background. In addition, pre-incubation with a cross-linking reagent often masks or prevents the antibody from binding to its antigen (J. Histochem. Cytochem. 45, 327 (1997); J. Histochem. Cytochem. 39, 741 (1991); J. Histochem. Cytochem. 43, 193 (1995); Appl. Immunohistochem. Molecul. Morphol. 9, 176 (2001)).
In a variation of this blocking strategy, a multi-step sequential-labeling procedure is used to overcome the problems of cross-reactivity. The sample is incubated with a first antibody to form a complex with the first antigen, followed by incubation of the sample with a fluorophore-labeled goat Fab anti-mouse IgG to label the first antibody and block it from subsequently complexing when the second antibody is added. In the third step, a second mouse antibody forms a complex with the second antigen. Being blocked from cross-reacting with the first antibody, the second mouse antibody is detected with a standard indirect-labeling method using a goat anti-mouse antibody conjugated to a different fluorescent dye (J. Histochem. Cytochem. 34, 703 (1986)). This process is complex in that it requires multiple incubation steps and washing steps and it still cannot be used with mouse antibodies to probe mouse tissue.
Another blocking method is disclosed in the animal research kit (ARK) developed by DAKO. In this kit, a primary antibody is complexed with biotin-labeled goat Fab anti-mouse IgG and excess free Fab is blocked with normal mouse serum. However, since the Fab used in this process is generated from the intact IgG (rather than a selected region) there is a potential for the formation of anti-paratope or anti-idiotype antibodies that will block the antigen-binding site and prevent immunolabeling. The biotinylated antibody also requires subsequent addition of a labeled avidin or streptavidin conjugate for its subsequent visualization.
The present invention is advantageous over previously described methods and compositions in that it provides the benefits of indirect labeling with the easy and flexibility of direct labeling for determination of a desired target in a biological sample. The present invention provides labeled monovalent proteins specific for a target-binding antibody, which are complexed prior to addition with a biological sample. Because these monovalent proteins are not bivalent antibodies, precipitation and cross-linking are not a problem. Therefore the compositions of the present invention can be used with immunologically similar monoclonal or polyclonal antibodies of either an identical isotype or different isotypes. The monovalent labeling proteins are specific for the Fc region of target-binding antibodies, these proteins will not interfere with the binding region of the primary antibody. In addition, the monovalent labeling proteins are not negatively affected by the presence of primary amines like BSA, gelatin, hybridoma culture supernatants or ascites fluid, thus primary antibodies present in these media can be effectively labeled with the labeling proteins of the present invention. Thus, the present invention provides numerous advantages over the conventional methods of immunolabeling.